Boar taint refers to the disagreeable, `urine like` odor and flavor associated with heated or cooked meat of some uncastrated pigs. Affected and unaffected pigs are not readily distinguishable using commercially viable methods, young male pigs have been castrated to prevent boar taint. However, castration has significant economic drawbacks. Castrated pigs have inferior carcass characteristics and have a lower feed efficiency. It is estimated that uncastrated pigs have a 5-10% improvement in lean meat production over castrated pigs. Castrating young pigs is also cumbersome and costly. Consequently, a substantial gain in productivity can be realized by using entire male pigs for pork production (deLange, C F M and Squires, E J, 1995). Therefore, methods to prevent or determine predisposition to boar taint, that do not require castration of young pigs, are needed.
The 16-androstene steroids, primarily 5.alpha.-androst-16-en-3-one (androstenone) have been associated with boar taint (Patterson, 1968; Bonneau, 1982), and levels of the steroids in fat have been used for estimating boar taint (Squires, 1990; Squires et al., 1991). The 16-androstene steroids are produced in the Leydig cells of the testis and then pass, via the spermatic vein, into the systemic circulation (Bonneau, 1982). Due to their hydrophobic nature, 16-androstene steroids are absorbed from the circulation by the fatty tissues. Subsequent volatization of the steroids during the heating process produces the off odor and flavor associated with boar taint.
The initial reaction in the formation of the 16-androstene steroids in the testis is catalyzed by the andien-.beta. synthase system (Katkov and Gower, 1970). Components of the andien-.beta. synthase system are: cytochrome b5, cytochrome P450c17, NADH cytochrome b5 reductase, NADPH cytochrome P450c17 reductase.
Cytochrome P450c17 converts C21 pregnenolone to C19, 5,16-androstadien-3.beta.-ol which is eventually converted into the pungent androstenone. Cytochrome P450c17 catalyses the formation of the androgen precursor dehydroepiandrosterone (DHEA) from pregnenolone via the C17,20-hydroxy/lyase reaction. Levels of the growth promoting androgens and, therefore, cytochrome P450c17 must remain unaltered in any attempt to reduce the levels of the 16-androstene steroids associated with boar taint.
Cytochrome b5 has been found to be important for the formation of the 16-androstene steroids in vitro (Meadus et al., 1993; Lee-Robichaud et al., 1995). High levels of cytochrome b5 have also been associated with high levels of adrenal steroidogenesis (Sakai et al., 1993). Cytochrome b5 is a small amphipathic hemeprotein which is capable of accepting and transferring a single electron (Velick and Strittmatter, 1956). Two isoforms of cytochrome b5 exist in mammals; a larger, 134 amino acid membrane-bound isoform and a smaller, 98 amino acid soluble isoform (Kimura et al., 1984; Abe et al., 1985). The soluble isoform lacks the C-terminal hydrophobic tail possessed by the membrane-bound isoform and contains only the N-terminal 98 amino acid catalytic domain whose sequence is identical to that of the membrane-bound isoform, except for the 98th amino acid which is different in the porcine isoforms (Cristiano et al., 1993).
In humans and rabbits, the soluble and microsomal isoforms of cytochrome b5 have been reported to originate from a single gene (Giordano and Steggles, 1993). Different isoforms of cytochrome b5 result from differential processing of the pre-mRNA transcript and the gene has been isolated and characterized in humans (Li et al.). Expression of the microsomal isoform has been reported to be ubiquitous, being present in all tissues examined (Giordano and Steggles, 1993). Soluble cytochrome b5 is found in erythrocytes and functions in vivo to reduce methemoglobin (Hulquist and Passon, 1971). Soluble cytochrome b5 mRNA has also been detected in lung, gall bladder, adrenal gland and bone marrow suggesting that soluble cytochrome b5 participates in other redox reactions (Giordano and Steggles, 1993). Most recently, soluble cytochrome b5 has been reported to be involved in the biosynthesis of the sialic acid, N-glycolylneuraminic acid in the liver (Kawano et al., 1994).